The tone of the passage can best be described as ______.

A．discussing the importance of solar energy
B．presenting a history of research on energy sources
C．describing a possible solution to the problem of the cost of photo-voltaic cells
D．advocating increased government funding for research on alternative energy source

A．unusual ways to advertise products
B．types of payment plans for service
C．theories about how products affect different levels of society
D．how certain elements of a price "package" influence its market value